Thermocouple



C. J. CARTER July 31, 1956 THERMOCOUPLE Filed March 10, 1955 INVENTOR. Charles J. Car/er gmcaew 03W 2,757,220 Patented July 31, 1956 lice 2,757,220 THERMOCOUPLE Charles 'J'. Carter, Milwaukee, Wis., assignor to General Electric Company, a corporation of NewYork ApplicationMarch10, 1955, SerialNo. 493,430

13' Claims. (Cl. 136-4) This invention relates to thermocouples and particularly to an improved construction for thermocouple junctions andthe, method of making the same.

Thermocouples in many applications must withstand extreme operating conditions. An example of a thermo couple application in which extreme operating conditions are encountered in the use to measure the temperature of the exhaust stream in aircraft gas turbine engines. Thermocouples in the exhaust stream of gas turbine eng'i'nes'are important control elements inthat the temperatitre sensed thereby is used to control the fuel injection into the combustion chambers so that the engine temperatures do notexceed certain critical values. The changes infthe-temperature of the jet exhaust stream: can be very rapid. A very short period of operation at temperatures above certain: critical temperatures canquickly burn out or destroy operating parts of the engine causing failure or; destruction of the engine. 7 v

The operating conditions encountered; in jet exhaust streams are severe since the velocity of the jet stream, is verv high. and: the temperature of the stream is. very high.

Although efforts are made to filter the air intake. to the engines, a certain amount of debris passes throughthe engine and causes erosion of all parts in the path of the jet stream including the temperature sensing junctions of thermocouples extending therein. Incomplete combustion" of the fuelcan also develop abrasive particles as wenas corrosive compounds which are thrown against and act upon the thermocouple junction in the jet exhaust stream. In orderpr'operly to perform as a part ofthe' fuel in jec'tion' control system, the thermocouples inthe jet exhaust stream must possess a good response rate so that even rapid changes in temperature are accurately detected and the.- presence} of these high temperatures communicated to the control devices so that the tempera.- ture of the jet stream is reduced by decreasing; the fuel input, before the engine parts are damaged'or burned out. Thetherrnocouples must. also be: able: to withstand severe thermal: shock since the temperature of the 'jet stream can change very rapidly particularly when the engine. is, started.

A type. of thermocouple particularly adapted for use in the application described is a thermocouple having as the active metal conductors, a Chromel electrode and an Alumel electrode suitably welded together to forma thermocouple junction. The portions of the thermocouple conductors or wires leading from the junction must be insulated from each other and from the mounting therefor and'to' this end the thermocouple wires are preferably packedin an insulating core such as of alumina or magnesia. Another problem is encountered in using thermocouples of this type in jet streams in that although thecore of insulating material is treated to make it impe-rviousto. fluids, the; interfaces between the insulating material and the thermocouple wires usually have. not been heretofore successfully sealed. a. result liquids including fuel tend to creep up the thermocouple wires on. the surface thereof and become trapped by the insulating material. of the core. This is particularly troublesome when rapidly. accelerating or deaccelerating since large amounts of unburned fuel may pass out with the exhaust stream at these times. The fuel absorbed between the thermocouple wires and the insulating core is soon carbonized by the high temperatures present to form a conductive material which greatly lowers the insulation' resistance and impairs the performance of the thermocouple.

Accordingly, it is an important object ofthe present invention to provide an improved thermocouple and particularlya thermocouple which is suitable for use in regions experiencing extreme operating conditions.

More specifically, it is an important object of the present invention to provide an improved. thermocouple of the type set forth in which the thermocouple junction is fully protected from the surrounding media and yet performs: its function in a satisfactory manner.

Another object of the invention is to provide a thermocouple having a protected. junction which, has a long operating lifewhen utilized to measure high temperatures in high velocity streams under corrosive and abrasive operating conditions.

Yet another object of the invention is to provide a thermocouple having a protected junction. of the type set forth whichvhas, a good response rate and a steady calibration characteristic.

Still another object of the invention is 'to provide a thermocouple having an enamel coated junction, the

enamel coating on the junction sealing the pointof access to the inner face between the thermocouple wires and the insulating materials surrounding the thermocouple wires in the thermocouple mount.

Yet another object of the invention is to provide an improved thermocouple including a pair of conductors of dissimilar metals forming a thermocouple junction, the junction being coated with an enamel composition which adheres well to both metals and to the junction thereof and has a coefficient of expansion comparable to both dissimilar metals.

Another object of the invention is to provide a thermocouple coated with a single uniform layer of fired ceramic applied in one step. I

Still another object of the invention is to provide a thermocouple of the type set forth having an enamel coated junction, the junction having good thermocouple operating characteristics in which the enamel coating is resistant to thermal shock and will provide long operating life under extreme operating conditions.

A further object of the invention is to provide an improved method for applying enamel coatings'to thermocouple junctions of the type set forth.

These and other objects and advantages of the present invention will be better understood from the following description when taken in conjunction with the accompanying drawing. In the drawing wherein like reference numerals have been utilized to designate like parts throughout:

Figure l is a view in longitudinal section through a thermocouple made in accordance with and embodying the principles of the present invention, the thermocouple being shown schematically connected to an indicating circuit; a

Figure 2 is an enlarged view in longitudinal section through the end of the thermocouple and the thermocouple junction, abutt-welded V-junction being illustrated; and

Figure 3 is a-view similar to Figure 2 showing the invention applied to a butt-welded loop junction.

Referring to the drawing and particularly to Figure 1 thereof, there is shown a high temperature thermocouple made in accordance with and embodying the principles of the present invention. The thermocouple assembly is generally designated by the numeral 10. and is mounted in an aperture in a wall 12 of a gas turbine cngine. For purposes of illustration, the thermocouple assembly will be shown and described as incorporated in such an engine but it is to be understood that the thermocouple can be used to advantage in any similar application where similar operating conditions are encountered. The operative conductors of the thermocouple are conductors 14 and 16 which are formed of dissimilar metals and joined at one extremity thereof at a junction 18. Junction 18 is illustrated as being of the butt-welded V-type in Figures 1 and 2. Conductor 14 is preferably formed from a nickel-chromium alloy consisting, for example, of 90% nickel and 10% chromium. Such an alloy is available under the trademark Chromel-P and is generally referred to as a Chromel conductor. The other conductor 16 is preferably formed from a nickel-aluminum alloy which may also contain small amounts of manganese and silicon and consisting, for example, of 95% nickel, 2% aluminum, 2% manganese, and 1% silicon, and is generally referred to as an Alumel conductor.

Surrounding conductors 14 and 16 is a substantially cylindrical housing 20 which is formed of any suitable metal such as stainless steel which will withstand the extreme operating conditions encountered. The upper end of housing 20 extends through the aperture in housing 12 and is flared outwardly as at 22 at its outer end to protect the insulated thermocouple cable 24 which receives the upper ends of conductors 14 and 16. A bushing 26 formed of brass or other malleable metal is provided to position cable 24 within housing 20 at the desired point.

Means is provided to mount housing 20 through the aperture in wall 12 in the form of a mounting flange 28 which is preferably welded to housing 20 and has an outwardly directed flange 30 formed thereon. Flange 30 4 One preferred method of forming the refractory core $8 is to mix a' small amount of relatively low melting cementitious material with the refractory powder. The powdered mixture is then packed in housing 20 and the lower end of housing 20 (the end adjacent junction 18) is suitably heated. The cementitious material fuses and binds the refractory powder to form a solid plug 50. The cementitious material may suitably consist of, for example, a small percentage of clay, glass forming substances, or other materials so long as they do not fuse at the temperatures encountered in the thermocouple installation nor require so high a fusing temperature as to injure the thermocouple housing 20 on the conductors 14-16 when plug 50 is formed. A plug 50 as formed in this manner extends for a suitable distance from the junction end of the housing, one-quarter of the length of the housing 20 being satisfactory, leaving the remain der of the refractory cement in a powderedstate.

Another way of hardening the refractory material 48 to form a plug 50 is to compact firmly the powder at point 50 without use of a cementitious material. This can be accomplished by packing the refractory material, preferably magnesia in the housing, temporarily closing off the junction end of the housing, and swaging or otherwise working the junction end portion of the housing to reduce its volume. Under the high pressures thus applied, the refractory material 48 at point 50 becomes a hard solid mass or plug in which conductors 14 and 16 are firmly imbedded. The refractory material in the remainder of the housing remains in a powdered state.

According to the present invention, the exposed portions of thermocouple conductors 14, and 16 including the junction 18 are enclosed within a fired enamel coating to protect them from the severe operating conditions encountered in gas turbine engine exhaust areas. More specifically, a fired enamel coating 52 is provided about the portions of conductors 14 and 16 extending beyond is adapted to overlie an externally threaded member 32 secured as by welding to wall 12. An internally threaded nut 34 engages the threads on member 32 and clamps flange 30 thereagainst. In this manner the thermocouple assembly 20 is fixedly mounted in position through the aperture in wall 12.

Terminals 36 and 38 are provided for the ends of thermocouple conductors 14 and 16, respectively, and these connect to a pair of conductors 40 and 42 formed of corresponding metals. Conductors 40 and 42 are connected to a reference junction 44 which is usually placed at a point protected from the high temperatures of the gas turbine engine and in certain instances an ice point or other exactly determined reference temperature may be used for greater accuracy. A milliammeter 46 is shown connected to the circuit to record the current flow caused by the difference in the temperatures between the hot junction 18 and the reference junction 44. Milliammeter 46 may have a temperature scale calibration or a more accurate measuring means such as a potentiometer may be used in place of the milliammeter 46.

Disposed within cylindrical housing 20 and about the thermocouple conductors 14 and 16 is a quantity of refractory insulating powder 48. Powder 48 preferably extends from the bushing 26 downwardly to the lower end of housing 20 and forms an insulating core about the conductors 14--16 insulating them from each other and from housing 20. A preferred material for powder 48 is alumina or magnesia to which a small amount of cementitious powder may be added.

plug 50 and about the junction 18. Coating 52 is continuous and unbroken and seals against the compacted and impervious plug 50 described above.

In order to obtain the desired characteristics of enamel 52, a plurality of separate and distinct frits are employed in its manufacture. It is preferred to employ three socalled stock frits which may be described as molybdenum silicate glasses. The first of these frits has a relatively high proportion of titanium oxide. A second frit is utilized which has a relatively high cobalt oxide content. A third frit is used having a relatively high barium oxide content.

The three individual frits are blended together without chemical interaction or fusion and the blend is then mixed with refractory materials, suspension materials and water to form the enamel slip. The slip is milled to a standard fineness after which its specific gravity and pick-up are adjusted. The slip is then applied to the exposed portions of the thermocouple conductors 14 and 16 which have been annealed in a slightly reducing atmosphere. This annealing operation removes a major portion of gases, such as hydrogen, which may have been present in the thermocouple conductors, and thus conditions the conductors for optimum adhesion of the ceramic thereto. The coated thermocouple conductors and junction are then fired at an elevated temperature in a slightly reducing atmosphere.

In order to illustrate further the composition and the method of applying the enamel coating of the present invention, the following specific example will be given. It is to be understood that this example is given for purposes of illustration and it is to be understood that various changes and modifications can be made therein as will be more fully pointed out hereinafter without departing from the spirit and scope of the present invention.

Example I Frit Number Ingredients 3 V 1. Percent Percent Percent by weight by weight by weight Silica; 48:0 40. 5 31. Borajx 12; 0 31. 0 24. 0 Aluminum Hydrate. 4; 5 5.0 2.0 Molybdenum'0xide. 2.0 I 3.0 3. 0 Bal ium'Oxide 2:0 0. 0. 35.0 (.lobaltO'xide-.. 0.0 2. 5- 0.0 Titanium Oxide l2. 5 0. 0 0. O Fluorspar. 12.0 6.0 0.0 Sodium Nitra 3:0 12.0 0.0 Zinc=Oxide' 4. 0 0.0 5.0

A composite blended frit is then prepared from frits Nos. 1, 2 and 3 described above by mixing together 20 parts by Weight of fn't No. 1, 5 parts by weight of frit No: 2 and 75 parts by weight of hit No. 3; One hundred parts by weight of the blended frits are then mixed with 50'par'ts by weight of silica, 30 parts by weight of. chromium oxide, 5 parts by weight of clay and 45 parts by weight of water. This mixture is then ground in a porcelain mill? to a fineness such that only 0 to 1% by weight is retained by a 325 mesh U. S. standard screen; The specific gravity of the slip is established at about 1.65 and the slip is adjusted toa pick-up of 50 grams wet on a square foot plate. This produces the enamel slip to be used in coating the exposed portions of the conductors of the thermocouples.

The thermocouples to be coated are then annealed in aslightly reducing atmosphere for one hour at1800 F.

the annealing, grease or other contaminants on the thermocouple wires is burned 0E. The atmosphere had the following composition by volume:

Percent Hydrogen 14.5 Carbon monoxide 106. Carbon dioxide 4.0 Methane 2.2 Nitrogen 68.7

Percent I-Iydrogen 11 Carbon monoxide 8.2 Carbon dioxide 6.2 Methane 1L5 Nitrogen 73.1

After firing, the enamel coating 52 on the exposed portions of the thermocouple conductors 14' and 16 was found to be continuous and firmly bonded to both the AIu'mel'and'Chromel conductors. Furthermore, a tight seal was found to have been made with the compacted core50'so that the interface between the conductors 14 and 16 and the core 50 was sealed at the lower end. This seal between the fir'e'd ceramic coating 52 and the core 50 prevents fuel and other liquids from being carried along the interface betweenthe conductors 14'16' and core 50. This is important since if fueldoesmove along 0on ductors-1416 into core 50; the fuel" is soon carbonized by the extreme operating temperatures encountered. This-results in a reduction of theresistance' of the insulation of core 50 and causes a partial shorting of the conductors 14'-16 to thehousing 20 which ismaintainedf at ground potential. This in turn gives a lower temperature indication than actually exists at the thermocouple junction andfas a result of this lower temperature indication, the fuel-to-air ratio may be increased to a point Where the temperature in the combustion chamber-and in the exhaust zone is actually beyond the safe operating range.

These excessive temperatures cancause extensive. damage to jet engine components. after a very short period of operation.

The coated thermocouple when. tested was found to possess a response rate substantially the same'as that of a thermocouple in. which the exposed conductors and thermocouple junction: are not. coated.

It further. was: found that the coated thermocouple made according't'o the present invention had a'high -insw lating resistance; This indicated' thatr the ceramic enamel coating 52 had no adverse effect. on the insulating: core 50'; This is important since many dielectric or insulating materials when subjected to high temperatures tend to become semi-conductive when approaching the fluid state. If the dielectric material surrounding the conductors 1446- beeome semi-conductive; then errors in temperature indication will occur since there will be leakage paths for current toground. The insulation resistance to ground ofv thethermocouple 10 was found to be good at the operating conditions encountered in use. i

The ceramic coating 52 was also found to be' highly resistant to thermal shock. The coating 52 adhered to both the conductors 14-16-'an'd' to the Welded junction 18 even when. extreme and suddenv changes in: tempera:- ture were applied thereto.

It was further found that the coated thermocouple junction. made' in accordance with therpresent' invention exhibited good calibration" characteristics in thatthe thermocouple junctions were readily reproducible. and maintained their calibration throughout the expected service life. It is believed a good. calibrating characteristic results at least in part from the annealing and other preparation of the conductors and. the welded junction before coating. Surface preparation and heat treatment" are believed to change the thermoelectric properties of the junction toward the positive side and that annealing in a reducing atmospherechanges the thermoelectric properties toward the negative side. The result of the combined operations is to produce a thermocouple which has good calibration characteristics which are retained throughout its operativelife.

I To summarize, it was found that the enamelcoating 52 adhered to all portions of the surface of the con" dilctors 1'4" and 16 and to the junction 18 as well as to the insulating core 50. There was noevidence of spall ingeven at the junction at any time. The coating sub.-' stantially eliminatedabsorption of liquid fuel into' the insulating core 50- around conductors 14 and 16. In addition, coating 52 protects the junction'18 and the thermocouple conductors from corrosive atmospheres encountered and from abrasion caused by particles in the exhaust stream; The coefiicient of expansionof the coating 52 is highly compatible with both the Alumel and Chrom'el conductors, which results in good resistance to thermal shock and a long operating life. The insulating resistance to ground'of the completed thermo couple was good and the calibration was substantially the same as that of an uncoatedthermocouple junction.

In the manufacture of these coated thermocouples for use in aircraft gas turbine engines, each thermocouple is tested with respect to each of the: character'- ist'ics described: above; and any thermocouple departing in any particular from the corresponding-narrow range I of the standard specification is rejected. For example, the permissiblerange of the response rate of these coated thermocouples in the standard specification is within a narrow band that does not depart more than 5% from that of the corresponding uncoated. thermocouples. a a I Allof the above advantages are achieved in these thermocouples by .a homogeneous, single layer of ceramic coating applied in one step. 'It further has been found that it is necessary to have the three separate frits Nos. 1, 2 and 3' present in the slip before firing in order The slightly reducing'atmosphere used in annealing the thermocouples prior. toceramic coating, may vary slightly in compositionfrom that disclosed in Examplel and still be satisfactory so long as the essentialreducing characteristics of the atmosphere are preserved." A' slightly lower or higher annealing temperature may be used provided that'the time of annealing, is correspondingly adjusted to achieve the degree of annealing described above- Similarly the firing or maturing atmosphere may have the composition thereof slightly altered providing the reducing character thereof is essentially maintained.

to obtain the desired characteristics of each, since it 1 also hasbeen found that a combined composition frit "having the same gross composition as the combined frits Nos. 1,2 and 3 does not produce'a satisfactory;

enamel coating 52.

three frits when coating thermocouples :for use in ex haust streams of aircraft gas turbines:

. Frit Number Ingredients Percent 7 Percent: Percent Silica 48. 52. 0 38. 0-42. 0 28.0-34.0 Borax 11. 0-15. 0 27.04331] 10. o30. 0 Aluminum Hydram. 4.0 6.0 4.0 6.0 1. 0- 3.0 Molybdenum Oxide 5- 4,. 5 5 4. 5 5- 4.5 Barium Oxide. 30- 0-40. 0 Cobalt Oxide Titanium 0 F1u0r'spar.'.. I Sodium Nitrate. Zinc Oxide The above ranges are based on estimates from partial analytical surveys of the frits utilized.

Although the optimum ratio by weight among frits Nos. 1, 2 and 3 were given above in Example 1, other proportions of these three frits can be used to achieve a good ceramic coating 52 in accordance with this invention. More specifically, it has been found that the amount of frit No. 1 used in forming the blended frit may be from about to about 40 parts by weight, the amount of frit No. 2 can be from about 2 to about parts by weight and the amount of frit No. 3 can be from about 60 to about 90 parts by Weight.

The 50 parts by weight of silica and the parts by weight of chromium oxide disclosed in Example I in the slip serve as refractory materials. Other refractory materials which might be substituted at least in part for the silica and the chromium oxide disclosed in Example I, include feldspar, calcined alumina, zirconium oxide and quartz. Instead of 50 parts by weight of silica in theslip, as little as parts or as much as 60 parts may be used. Instead of 30 parts by weight of chromium oxide as little as 20 parts and as much as 50 parts may be used.

The clay in the composition of Example I acts as a suspension agent to keep the frit particles in suspension. It has been found that as little as three parts by Weight of clay may be used or as many as eight parts by weight of clay may be used in the disclosed composition and still obtain a good ceramic coating 52 on the thermocouple. Other suspension materials which may be used to replace part or all of the clay disclosed in Example I include sodium nitrite, potassium nitrite and bentonite.

The slip is preferably adjusted to a specific gravity from between 1.65 to about 1.68. It is then allowed to set to a pick-up of from about to about 55 grams Wet on a square foot plate.

7 in accordance with the present invention. in Figure 3 of the drawings, a thermocouple junction formed of Chromel and Alumel conductors 14 and 16. The junction designated by the numeral 54 is butt-welded A ceramic coating 56 like coating The firing temperature may be slightly greater or slightly less provided the firing time is appropriately adjusted.

The thickness of the ceramic coating is preferably about 0.001 inch but slightly less or greater thicknesses can be used and up to as much as 0.002 inch.

Other shapes of thermocouple junctions can be coated There is shown and is of the loop type. 52 described above is shown applied around the portions of conductors 14 and 16 which extend beyond the insulating core and about the thermocouple junction 54. Ceramic coating 56 seals against the insulating core 50 whereby to seal the interface between the conductors '14'-16 andthe insulating material 50 to prevent access of fuel thereto. The ceramic coated loop-junction of Figure 3 possesses all of the advantages described above with respect to theceramic coated V-junction 18.

It will be seen that there has been provided a ceramic coating composition for thermocouples and a method of applying the ceramic coating which fulfills all of the objects and advantages described above. 7 Although certain preferred forms of the invention have been given for purposes of illustration, it is to be understood that various changes and modifications can bemadetherein without departing from the spirit and scope of the invention.

What is claimed is: j j I a l. A thermocouple'for'use in measuring temperatures comprising a pair of conductors of dissimilar metals provided with a thermocouple junction adjacent to the ends thereof and coated with a layer of heat resistant and corrosion resistant enamel intimately bonded thereto and consisting essentially of a tired enamel slip; said slip comprising by weight about parts of blended frits, about 60 to parts of refractory materials, and about 3 to 8 parts of suspension agents; said blended frits comprising by weight about 10 to 40 parts of frit No. 1, about 2 to 20 parts of frit No. 2, and about 60 to 90 parts of frit No. 3; said frit No. 1 comprising by Weight about 48.0% to 52.0% silica, 11.0 to 15.0% borax, 4.0 to 6.0% aluminum hydrate, 0.5 to 4.5 molybdenum oxide, 1.0 to 3.0% barium oxide 10.0 to 15.0% titanium oxide, 10.0 to 20.0% fluorspar, 2.5 to 4.5% sodium nitrate, and 3.0 to 6.0% zinc oxide; said frit No. 2 comprising by weight about 38.0 to 42.0% silica, 27.0 to 33.0% borax, 4.0 to 6.0% aluminum hydrate, 0.5 to 4.5% molybdenum oxide, 0.5 to 3.0% cobalt oxide, 2.0 to 10% fiuorspar, and 8.0 to 15.0% sodium nitrate; and said frit No. 3 comprising by weight about 28.0 to 34.0% silica, 10.0 to 30.0% borax, 1.0 to 3.0% aluminum hydrate, 0.5 to 4.5% molybdenum oxide, 30.0 to 40.0% barium oxide, and 2.0 to 7.0% zinc oxide.

2. A thermocouple as set forth in claim 1; wherein the refractory materials are selected from the group consisting of silica, chromium oxide, feldspar, calcined alumina, zirconium oxide, and quartz; and the suspension agents are selected from the group consisting of clay, alkali metal nitrites, and bentonite.

3. A thermocouple as set forth in claim 1; wherein the refractory materials comprise by weight about 40 to 60 parts of silica, and about 20 to 50 parts of chromium oxide; and the suspension agent is clay.

avera e 4. A thermocouple as set forth in claim 1'; wherein the refractory materials comprise by weight about 50 parts ofsili'ea, and about 30 parts ofchromium oxideythe suspensionagent comprises by Weight about par-ts of clay; and the blended frits comprise by weight about 20' parts" of frit No. 1, about 5 parts of frit No. 2', and about 75 parts of frit No. 3.

5. The method of. applyinga fired enamel coating to a thermocouple junction comprising the steps of annealing the: uncoated thermocouple junction. at an elevated temperature in a reducing atmosphere; thereafter freshening the surface of the thermocouple junction, applying a coating of enamel slip to the freshened thermocouple junction, and firing the coated thermocouple junction at an elevated temperature in a reducing atmosphere to produce a ceramic coating thereon; said enamel slip comprising by weight about 100 parts of blended frits, about 60 to 110 parts of refractory materials, and about 3 to 8 parts of suspension agents; said blended frits comprising by weight about 10 to 40 parts of frit No. 1, about 2 to 20 parts of frit No. 2, and about 60 to 90 parts of frit'No. 3; said frit No. 1 comprising by weight about 48.0% to 52.0% silica, 11.0 to 15.0% borax, 4.0 to 6.0% aluminum hydrate, 0.5 to 4.5% molybdenum oxide, 1.0 to 3.0% barium oxide, 10.0 to 15.0% titanium oxide, 10.0 to 20.0% fluorspar, 2.5 to 4.5% sodium nitrate, and 3.0 to 6.0% zinc oxide; said frit No. 2 comprising by weight about 38.0 to 42.0% silica, 27.0 to 33.0% borax, 4.0 to 6.0% aluminum hydrate, 0.5 to 4.5% molybdenum oxide, 0.5 to 3.0% cobalt oxide, 2.0 to 10.0% fluorspar, 8.0 to 15.0% sodium nitrate; and said frit No. 3 comprising by weight about 28.0 to 34.0% silica, 10.0 to 30.0% borax, 1.0 to 3.0% aluminum hydrate, 0.5 to 4.5% molybdenum oxide, 30.0 to 40.0% barium oxide, and 2.0 to 7.0% zinc oxide.

6. The method of applying a fired enamel coating as set forth in claim 5; wherein the annealing step is carried out in a reducing atmosphere comprising by volume about 14.5 parts hydrogen, 10.6 parts carbon monoxide, 4.0 parts carbon dioxide, 2.2 parts methane, and 68.7 parts nitrogen; and wherein the reducing atmosphere used during the firing step comprises by volume about 11 parts hydrogen, 8.2 parts carbon monoxide, 6.2 parts carbon dioxide, 1.5 parts methane, and 73.1 parts nitrogen.

7. The method of applying a fired enamel coating as set forth in claim 6, wherein the annealing step is carried out at a temperature of about 1900 F. in an atmosphere having a dew point of about 80 F., the freshening of the thermocouple junction comprises sandblasting the same with about 30 grit sand at a pressure of about 15 lbs. per square inch, and the firing step is carried outat a temperature of about 1925 F. while the reducing atmosphere is maintained at a dew point of about 40 F.

8. The method of applying a fired enamel coating as set forth in claim 5, wherein the annealing step is carried out at about 1900 F. for about one hour, and the firing step is carried out at about 1925 F. for about twenty minutes.

9. The method of applying a fired enamel coating to a thermocouple junction as set forth in claim 5, wherein the slip is adjusted to a specific gravity of about 1.65 to 1.68 and set to a pick-up of 45 to 55 grams weight on a square foot plate, and the slip is applied to the thermocouple junction in amount such that the fired coating has a thickness of about 0.001 inch to 0.002 inch.

10. The method of applying a fired enamel coating to a thermocouple junction comprising the steps of annealing the uncoated thermocouple junction at a temperature of about 1900 F. for about sixty minutes in a reducing atmosphere comprising by volume about 14.5 parts hydrogen, 10.6 parts carbon monoxide, 4.0 parts carbon dioxide, 2.2 parts methane, and 68.7 parts nitrogen at a dew point of about 80 F., thereafter sandblasting the thermocouple junction with about 30 grit sand at a pressure of about 15 pounds per square inch, applying to the thermocouple junction a: coating of" enamel slip having" a spec'ific-- gravity of? about 1.65 to 1168 and set to' a'pi'ck up of about 45' to 55 grams weight on a square foot plate" and milledftda fineness of about 1% retained by a 325 mesh Si standard screen, the thickness of the slip on the thermocouple jun'ctio'n being such' that the fii'ed' co'ating has a thickness of. about 0.001 inch to 0.002 inch, and thereafter firing the' coated thermocouple junction at atern:- pera'ture of about 1925 F. for about twenty. minutes? in an atmo's'phere comprising by volume about p ns hydrogen, 8.2 parts carbon monoxide, 632" parts carbon dioxide, 1.5 parts methane, and 73.1 parts nitrogen at a dew point of about 40 F.; said enamel slip comprising by weight about 100 parts of blended frits, 60 parts of silica, 30 parts of chromium oxide, 5 parts of clay; said blended frits comprising by weight about 20 parts of frit No. 1, about 5 parts of frit No. 2, and about parts of frit No. 3; said frit No. 1 comprising by weight about 48.0% to 52.0% silica, 11.0 to 15.0% borax, 4.0 to 6.0% aluminum hydrate, 0.5 to 4.5% molybdenum oxide, 1.0 to 3.0% barium oxide, 10.0 to 15.0% titanium oxide, 10.0 to 20.0% fluorspar, 2.5 to 4.5% sodium nitrate, and 3.0 to 6.0% zinc oxide; said frit No. 2 comprising by weight about 38.0 to 42.0% silica, 27.0 to 33.0% borax, 4.0 to 6.0% aluminum hydrate, 0.5 to 4.5% molybdenum oxide, 0.5 to 3.0% cobalt oxide, 2.0 to 10.0% fluorspar, and 8.0 to 15 .0% sodium nitrate; and said frit No. 3 comprising by weight about 28.0 to 34.0% silica, 10.0 to 30.0% borax, 1.0 to 3.0% aluminum hydrate, 0.5 to 4.5% molybdenum oxide, 30.0 to 40.0% barium oxide, and 2.0 to 7.0% zinc oxide.

11. The method of applying a fired enamel coating to a thermocouple junction comprising the steps of annealing the uncoated thermocouple junction at an elevated temperature in a reducing atmosphere, thereafter sandblasting the thermocouple junction, applying a coating of enamel slip to the thermocouple junction to a thickness of about 0.001 inch to 0.002 inch, and firing the coated thermocouple junction at an elevated temperature in a reducing atmosphere to produce a ceramic coated junction; said enamel slip comprising by weight about 100 parts of blended frits, about 60 to 110 parts of refractory materials, and about 3 to 8 parts of suspension agents; said blended frits comprising by weight about lO to 40 parts of a high titanium oxide frit, about 2 to 20 parts of a high cobalt oxide frit, and about 60 to parts of a high barium oxide frit.

12. A ceramic coating slip comprising by weight about parts of blended frits, about 60 to parts of refractory materials, and about 3 to 8 parts of suspension agents; said blended frits comprising by weight about 10 to 40 parts of frit No. 1, about 2 to 20 parts of frit No. 2, and about 60 to 90 parts of frit No. 3; said frit No. 1 comprising by weight about 48.0% to 52.0% silica, 11.0 to 15.0% borax, 4.0 to 6.0% aluminum hydrate, 0.5 to 4.5% molybdenum oxide, 1.0 to 3.0% barium oxide, 10.0 to 15.0% titanium oxide, 10.0 to 20.0% fluorspar, 2.5 to 4.5% sodium nitrate, and 3.0 to 6.0% zinc oxide; said frit No. 2 comprising by weight about 38.0to 42.0% silica, 27.0 to 33.0% borax, 4.0 to 6.0% aluminum hydrate, 0.5 to 4.5 molybdenum oxide, 0.5 to 3.0% cobalt oxide, 2.0 to 10.0% fluorspar, and 8.0 to 15.0% sodium nitrate; and said frit No. 3 comprising by weight about 28.0 to 34.0% silica, 10.0 to 30.0% borax, 1.0 to 3.0% aluminum hydrate, 0.5 to 4.5% molybdenum oxide, 30.0 to 40.0% barium oxide and 2.0 to 7.0% zinc oxide.

13. A ceramic coating slip comprising by weight about 100 parts of blended frits, about 50 parts of silica, about 30 parts of chromium oxide, and about 5 parts of clay; said blended frits comprising by weight about 20 parts of frit No. 1, about 5 parts of frit No. 2, and about 75 parts of frit No. 3; said frit No. 1 comprising by weight about 48.0 to 52.0% silica, 11.0 to 15.0% borax, 4.0 to 6.0% aluminum hydrate, 0.5 to 4.5% molybdenum oxide,

1.0 to 3.0% barium oxide, 10.0 to 15.0% titanium oxide, 10.0 to 20.0% fluorspar, 2.5 to 4.5% sodium nitrate, and 3.0 to 6.0% zinc oxide; said frit No.2 comprising by weight about 38.0 to 42.0% silica, 27.0 to 33.0% borax, 4.0 to 6.0% aluminum hydrate, 0.5 to 4.5% molybdenum 5 oxide, 0.5 to 3.0% cobalt oxide, 2.0 to 10.0% fluorspar, and 8.0 to 15.0% sodium nitrate; and said frit N0. 3 comprising by weight about 28.0 to 34.0% silica, 10.0 to 30.0% borax, 1.0 to 3.0% aluminum hydrate, 0.5 to 4. 5% molybdenum oxide, 30.0 to 40.0% barium oxide, 10 and 2.0 to 7.0% zinc oxide.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A THERMOCOUPLE FOR USE IN MEASURING TEMPERATURES COMPRISING A PAIR OF CONDUCTORS OF DISSIMILAR METALS PROVIDED WITH A THERMOCOUPLE JUNCTION ADJACENT TO THE ENDS THEREOF AND COATED WITH A LAYER OF HEAT RESISTANT AND CORROSION RESISTANT ENAMEL INTIMATELY BONDED THERETO AND CONSISTING ESSENTIALLY OF A FIRED ENAMEL SLIP; SAID SLIP COMPRISING BY WEIGHT ABOUT 100 PARTS OF BLENDED FRITS, ABOUT 60 TO 110 PARTS OF REFRACTORY MATERIALS, AND ABOUT 3 TO 8 PARTS OF SUSPENSION AGENTS; SAID BLENDED FRITS COMPRISING BY WEIGHT ABOUT 10 TO 40 PARTS OF FRIT NO. 1, ABOUT 2 TO 20 PARTS OF FRIT NO. 2, AND ABOUT 60 TO 90 PARTS OF FRIT NO. 3; SAID FRIT NO. 1 COMPRISING BY WEIGHT ABOUT 48.0% TO 52.0% SILICA, 11.0 TO 1K.0% BORAX, 4.0 TO 6.0% ALUMINUM HYDRATE, 0.5 TO 4.5% MOLYBENUM OXIDE, 1.0 TO 3.0% BARIUM OXIDE 10.0 TO 15.0% TITANIUM OXIDE, 10.0 TO 20.0% FLUORSPAR, 2.5 TO 4.5% SODIUM NITRATE, AND 3.0 TO 6.0% ZINC OXIDE; SAID FRIT NO. 2 COMPRISING BY WEIGHT ABOUT 38.0 TO 4I.0% SILICA, 27.0 TO 33.0% BORAX, 4.0 TO 6.0% ALUMINUM HYDRATE, 0.5 TO 4.5% MOLYBDENUM OXIDE, 0.5 TO 3.0% COBALT OXIDE, 2.0 TO 10% FLUORSPAR, AND 8.0 TO 15.0% SODIUM NITRATE; AND SAID FRIT NO. 3 COMPRISING BY WEIGHT ABOUT 2,.0 TO 34.0% SILICA, 10.0 TO 30.0% BORAX, 1.0 TO 3.0% ALUMINUM HYDRATE, 0.5 TO 4.5% MOLYBDENUM OXIDE, 30.0 TO 40.0% BARIUM OXIDE, AND 2.0 TO 7.0% ZINC OXIDE. 